nvme-tcp: have queue prod/cons send list become a llist

The queue processing will splice to a queue local list, this should
alleviate some contention on the send_list lock, but also prepares
us to the next patch where we look on these lists for network stack
flag optimization.

Remove queue lock as its not used anymore.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Tested-by: Mark Wunderlich <mark.wunderlich@intel.com>
[hch: simplified a loop]
Signed-off-by: Christoph Hellwig <hch@lst.de>